Many devices in modern life require renewable energy sources for their operation. Among the more well-known of such devices are portable computers, electric vehicles, and medical prostheses. The importance of a reliable energy source for such devices is illustrated below taking medical prostheses as a specific example.
Many different kinds of medical prostheses have been implanted in hundreds of thousands of people every year. These devices can help to relieve pain, restore lost body functions, and even extend life itself. Examples of these devices are artificial hearts, pacemakers, hearing aids, drug delivery systems, nerve stimulators, and bone-growth stimulators.
It is highly desirable that these implanted devices be self-contained and self-sufficient to obviate the need for percutaneous energy transmission to the implanted device. A common method of providing energy to these devices is to use batteries.
A problem with batteries, however, is that they discharge to the point of inoperability after given periods of use. Thereafter, the batteries must either be replaced, or in the case of rechargeable batteries, recharged in order to continue operation of the device powered by the batteries.
"Storage" batteries can be recharged, with the state of charge being typically determined either roughly by a voltage test, which assumes that the battery output degrades steadily with time, or more precisely by a hygrometer which measures the specific gravity of the battery fluid which changes with use.
In many types of batteries, however, such methods of determining charge state are not possible. For example, the voltage output in nickel-cadmium (NiCd) batteries does not degrade steadily, but rather is substantially constant until complete discharge. Further, greater accuracy than that provided by a voltage test is often required. Since NiCd batteries are hermetically sealed, it is not possible or extremely inconvenient to measure the liquids (if any) in the battery.
Further, it is well known that a battery's capabilities change with variations in temperature, and the charge state of the battery must be determined taking the temperature environment into consideration.
One way of precisely determining the charge state is to allow the battery to run down to a zero charge state. This presents obvious problems of maintenance of the performance of the device being energized by the battery. This method clearly cannot be used in medical device applications as it presents a serious danger to patients. Solutions to the problem have included a signal warning to the patient requiring the patient to connect another energy source. This presents a problem for disabled or sleeping patients. Another solution is a monitoring computer which can be utilized to automatically connect another source of energy. This, however, presents other problems of device bulkiness and complexity as well as a need for a power source for the computer which itself may also run out.
As mentioned above, batteries such as the NiCd type, have a battery output that is substantially constant until just before running out, thereby giving little or no warning before complete discharge. In fact, batteries having such a characteristic output curve are desirable because gradual decreases of output indicate an increase in battery resistance which in turn indicates an inefficient battery which is not suitable for many purposes. Thus NiCd batteries offer the desirable characteristic of a stable output (which is necessary in many devices such as implanted prostheses), but that very stability makes it almost impossible to know the state of charge from a voltage output measurement.
One example of a prior art solution to these problems is U.S. Pat. No. 3,617,850 to Domshy which teaches a battery-status device utilizing the past history of the battery. If the amount of energy provided by a battery is recorded and the battery is recharged by pumping in a comparable amount of energy, a rough estimate is produced that the a battery is fully charged. It is only an estimate because as the battery ages and its operating temperature changes, the battery will not accept charge at the same efficiency and therefore is likely not in the fully charged state. Domshy alters the charge status indicator based on the charge/discharge history of the battery to compensate for the age of the battery and makes a temperature measurement to compensate for temperature changes.
Domshy's device has the disadvantage of requiring a number of sensors and recording devices in order to provide the necessary complete discharge/charge history of the battery. This may not be possible in many applications, particularly those of medical prostheses, and the additional components increase the complexity of the device.
Other prior art devices and methods require applying a predetermined load on the battery to determine status. An example of this kind of device, primarily useful for automobile batteries, is U.S. Pat. No. 4,423,378 to Marino et al.
In electronic medical prosthetic devices, the battery in the device is typically in a body-implant system where applying loads and making external measurements is extremely difficult if not impossible.
Finally, in order to efficiently charge a battery the state of charge should be known so that time and energy are not wasted in continuing to charge a battery which has already reached its charge acceptance limit.
Thus there is a distinct need for knowledge of the state of charge of a battery which can be determined without the necessity of intrusive procedures or added device complexity requiring continual monitoring and recording.